Micro relief element and preparation thereof

ABSTRACT

A micro relief element which comprises 
     a) a first layer of a first substrate, the first layer having a receptive surface capable of retaining a relief forming polymer; 
     b) an overlay of a desired thickness of the relief forming polymer over the receptive surface; and 
     c) at least one relief feature formed from the relief forming polymer and which protrudes above the overlay; structures and elements comprising such micro relief element; micro-optical, micro-fluidic, micro-electrical and micro-chemical applications thereof; and a method and apparatus for the preparation thereof.

This invention relates to a micro relief element (MRE) and a method ofpreparing same.

An MRE, as referred to herein, is a 3-dimensional structure which isformed on the surface of a desired substrate and which structure is ableto perform a specific function. Typically, the structure is a repetitivepattern which protrudes above the substrate to a defined height of theorder of 0.1 to 1000 microns. Such an MRE can be used as an activecomponent in micro-optic, micro-fluidic, micro-electrical andmicro-mechanical devices. In particular, such an MRE can be used as amicro-optical element (MOE) and in which case the structure may be of aheight in the range 0.1 to 1000 microns, more commonly in the range 0.1to 10 microns. Where the MRE is a component in a micro-fluidic ormicro-mechanical device then the structures are usually of heights inthe range 10 to 1000 microns.

An MOE comprises a surface relief structure whose purpose is to inducephase changes on a light beam which is incident upon the structure suchthat a predetermined spatial distribution of the light results when theincident light is viewed either in reflection or transmission. MOEs alsoinclude structures in which the relief structure is embedded within alight transmissive material, hereinafter an immersed MOE, such as forexample an immersed microlens.

MOEs may be used for a variety of applications, such as diffractiongratings, lenses, beam array generators, laser harmonic separators,focusing mirrors and microlens arrays.

Microlens arrays can be used for optical readers, interfaces betweenlaser diodes and optical fibres, diffuser screens, integral photography,3-d camera and display systems, integrated optical devices andimagebars.

Usually, an MOE is formed by exposing and developing the desired surfacerelief structure into a photosensitive material coated onto thesupporting substrate and then transfering the surface relief structureinto the substrate by plasma or chemical etching. The conventionaldesign and fabrication of MOEs is discussed in “Synthetic diffractiveelements for optical interconnects”, M R Taghizadeh et al, OpticalComputing and Processing, Vol 2(4), pp 221-242, 1992; “Two-dimensionalarray of diffractive microlenses fabricated by thin film deposition”, JJahns et al, Appl Opt, Vol 29(7), 931, 1990; “Continuous-reliefdiffractive optical elements for two-dimensional array generation”, M TGale et al, Appl Opt, Vol 32(14), 2526, 1993; “Multilevel-grating arraygenerators: fabrication error analysis and experiments”, J M Miller etal, Appl Opt, Vol 32(14), 2519, 1993; and “Fabricating binary optics ininfrared and visible materials” M B Stern et al, SPIE, Vol 1751,Miniature and micro-optics, pp 85-95, 1992.

Microlens arrays have in the past been produced by different methods asdescribed in “Polymer microlens arrays”, P Pantelis and D J McCartney,Pure Appl.Opt., Vol 3, 103 (1994); “The manufacture of microlenses bymelting photoresist”, D Daley, R F Stevens, M C Hutley and N Davies,Meas. Sci. Technol., Vol 1, 759 (1990); and “Microlens array fabricatedin surface relief with high numerical aperture”, H W Lau, N Davies, MMcCormick, SPIE Vol 1544 Miniature and Micro-optics: Fabrication andSystem Applications, p178 (1991). Glass microlenses have been made bychemically etching glass, moulding glass, plasma etching glass toproduce a surface relief structure.

Polymer microlenses have been produced by melting islands of photoresistor by direct writing photosensitive materials with a laser beam or bydirectly writing a suitable material with an electron beam or by plasmaetching or by moulding.

Unfortunately, conventional methods of fabrication for MREs are limitedin the range of substrates that can be used and in the complexity andaccuracy of the relief structures that can be formed.

It is an object of the present invention to provide a facile method forproducing MREs, in particular MOEs, in a variety of substrates andcomplexity of designs. An advantage of the present method is that a widerange of heights of surface relief can be produced using the sameprocess. Another advantage is that small lateral features can besuccessfully reproduced. Additionally, the process may be used toproduce large area MREs.

Accordingly in a first aspect the present invention provides a microrelief element which comprises

a) a first layer of a first substrate, the first layer having areceptive surface capable of retaining a relief forming polymer;

(b) an overlay of a desired thickness of the relief forming polymer overthe receptive surface; and

(c) at least one relief feature formed from the relief forming polymerand which protrudes above the overlay.

In a second aspect the present invention provides a structure for use asat least part of a micro-optical element, which structure comprises

(a) a first layer of an optically transmissive first substrate having afirst refractive index, the first layer having a receptive surfacecapable of retaining an optically transmissive relief forming polymer;

(b) an overlay having an optically insignificant effect, preferablyhaving a maximum thickness of less than 1.5 μm, of the relief formingpolymer over the receptive surface, the relief forming polymer having asecond refractive index which is the same as or different from the firstrefractive index; and

(c) at least one optically active relief feature formed from the reliefforming polymer and which protrudes above the overlay.

In a third aspect of the present invention there is provided an immersedMOE comprising

3(a) a first layer of an optically transmissive first substrate having afirst refractive index, the first layer having a receptive surfacecapable of retaining an optically transmissive relief forming polymer;

(b) an overlay having an optically insignificant effect, preferablyhaving a maximum thickness of less than 1.5 μm, of the relief formingpolymer over the receptive surface, the relief forming opticallytransmissive polymer having a second refractive index which is the sameas or different from the first refractive index;

(c) at least one optically active relief feature formed from the reliefforming polymer and which protrudes above the overlay; and

(d) a second layer of an optically transmissive second substrate havinga third refractive index which is superimposed upon the at least oneoptically active relief feature and wherein not all of the first, secondand third refractive indices are the same.

In a fourth aspect of the present invention there is provided a methodof preparing a micro relief element which comprises

a) a first layer of a first substrate, the first layer having areceptive surface capable of retaining a relief forming polymer;

(b) an overlay of a desired thickness of the relief forming polymer overthe receptive surface; and

(c) at least one relief feature formed from the relief forming polymerand which protrudes above the overlay

 which method comprises

(a) forming a line of contact between the receptive surface and at leastone mould feature formed in a flexible dispensing layer;

(b) applying sufficient of a resin, capable of being cured to form therelief forming polymer, to substantially fill the at least one mouldfeature, along the line of contact;

(c) progressively contacting the receptive surface with the flexibledispensing layer such that

(1) the line of contact moves across the receptive surface;

(2) sufficient of the resin is captured by the mould feature so as tosubstantially fill the mould feature; and

(3) no more than a quantity of resin capable of forming the overlaypasses the line of contact;

(d) curing the resin filling the at least one mould feature so as toform the at least one relief feature; and, optionally, thereafter

(e) releasing the flexible dispensing layer from the at least one relieffeature.

In a fifth aspect of the present invention there is provided a method ofpreparing a structure for use as at least part of a micro-opticalelement, which structure comprises

(a) a first layer of an optically transmissive first substrate having afirst refractive index, the first layer having a receptive surfacecapable of retaining an optically transmissive relief forming polymer;

(b) an overlay having an optically insignificant effect, preferablyhaving a maximum thickness of less than 1.5 μm, of the relief formingpolymer over the receptive surface, the relief forming polymer having asecond refractive index which is the same as or different from the firstrefractive index; and

(c) at least one optically active relief feature formed from the reliefforming polymer and which protrudes above the overlay which methodcomprises

(a) forming a line of contact between the receptive surface and at leastone mould feature formed in a flexible dispensing layer;

(b) applying sufficient of a resin, capable of being cured to form therelief forming polymer, to substantially fill the at least one mouldfeature, along the line of contact;

(c) progressively contacting the receptive surface with the flexibledispensing layer such that

(1) the line of contact moves across the receptive surface;

(2) sufficient of the resin is captured by the mould feature so as tosubstantially fill the mould feature; and

(3) no more than a quantity of resin capable of forming the overlaypasses the line of contact;

(d) curing the resin filling the at least one mould feature so as toform the at least one optically active relief feature; and, optionally,thereafter

(e) releasing the flexible dispensing layer from the at least oneoptically active relief feature.

An MRE of the present invention may be capable of use as an activecomponent in a micro-optic, micro-fluidic, micro-electrical ormicro-mechanical device. However, the principle use herein envisaged foran MRE of the present invention is as a micro-optical element (MOE).Reference herein to features making up an MOE according to the inventionmay be to features which are equally advantageous in other applicationsof MRE's and references to MOE's will be construed as referring to MRE'saccordingly.

Such an MOE may be able to perform more than one optical function, e.g.an MOE for use as a beam corrective optic for diode lasers may combinethe functions of astigmatism correction, elipticity correction and beamcollimation.

Moreover, the optically active relief feature in combination with thesupporting first layer may be able to perform more than one opticalfunction, for example an optically active relief feature supported on ashaped first layer, suitably of lens shape, may provide for correctionof chromatic aberration.

Accordingly it will be apparent that the first layer and indeed the MREor MOE, and the relief feature(s) may be of any desired geometryaccording to the desired function to be performed. For example the firstlayer, including an optional support substrate, may be planar, hollow orsolid cylindrical, or may comprise a lens or other optical componentwherein the relief feature(s) is/are suitably applied to a surfacethereof. Alternatively or additionally the relief feature(s) may forexample comprise one or more continuous, stepped or otherwise profiledstructures such as lens, straight or angled track or lateral, annularring, straight or curved diffraction grating, multiple faced(pyramidal), or other optical, fluidic, electrical or mechanicalstructure.

Additionally, the MOE may be coated with an other material in order toprotect the MOE (anti-scratch coating) or to reduce reflection from theMOE (anti-reflection coating). Preferably, such coatings aremultilayered coatings.

Furthermore, the MOE may function in reflection rather thantransmission. This might be achieved by fabricating the MOE using areflective first layer or by coating the surface of the MOE to enhancereflection from it.

The first layer may be supported by a suitable support substrate whichmay be subsequently removed from the first layer. However, it ispreferred that the first layer is self-supporting or is associated witha support surface of desired geometry for a desired application.Suitably the first layer is comprised of any suitable material for theintended application which may be known in the art for example it may bea polymer film (in particular a film formed from polyester, such as PETor PEN, or an other polymer such as PVC, polyimide, PE or a knownbiodegradable polymer, e.g. poly(hydroxy butyrate)); a material selectedfor its optical transparency at certain wavelengths for example ZnSe orGermanium which are capable of operation in the infra-red region between2 and 15 micron; silicon; high temperature resistant inorganic metaloxide or ceramic such as titania or (fused) silica, e.g. glass; or itmay be a natural or synthetic paper product such as a wood pulp orsynthetic card or paper.

For certain applications, for example where semiconductor components aremounted onto the MRE and from which it is desirable to dissipate heat,the first layer may be coated with a layer of diamond or similarmaterial with a high thermal conductivity.

Additionally, the first layer may be coated with an electricallyconducting layer, e.g. indium tin oxide (ITO) or gold, so that anelectrical contact can be made to a semiconductor component located onthe surface of the first layer.

The receptive surface of the first layer may be coated with a suitablebonding agent, e.g. where the first layer is of glass, a silane couplingagent, which serves to more firmly anchor the relief feature to thefirst layer.

Coating of the first layer may be achieved as a continuous layer priorto forming the optically active relief structure(s) thereon, but isadvantageously achieved as a layer about the optically active reliefstructure(s), which may be created by replication from the flexibledispensing layer during the formation of the optically active reliefstructure(s).

The second layer may also be supported by a suitable, optionallyreleasable, substrate. The second layer may be superimposed on the atleast one optically active relief feature by any suitable means, e.g.lamination. The second layer may also be provided with at least onemould feature in which is moulded an optically transmissive polymer,which may be the same as the optically transmissive relief formingpolymer retained on the receptive surface, and which may be so placedthat at least some of the mould features of the second layer are matchedwith at least some of the mould features of the first layer such thatthey can form a composite optical component.

The selection of the relief forming polymer will be dependent on theintended use of the MRE and includes silica filled, light curable resinssuch as those used in dentistry and those for rapid prototyping bystereolithography, UV curable liquid crystal resins, photocationic epoxyresins and those optically transmissive resins as described below.

When optically transmissive, the relief forming polymer may be selectedfrom those known in the art including those developed as light curableadhesives for joining optical components for example those sold underthe name LUXTRAK (LUXTRAK is a tradename of Zeneca plc), those developedfor polymer optical fibre fabrication and those developed for opticalrecording using polymer photoresists. In particular the opticallytransmissive relief forming polymer may be formed from a suitable resinfor example halogenated and deuterated siloxanes, styrenes, imides,acrylates and methacrylates such as ethyleneglycol dimethacrylate,tetrafluoropropylmethacrylate, pentafluorophenylmethacrylate,tetrachloroethylacrylate, multifunctional derivatives of triazine andphosphazene. Resins and polymers that contain highly fluorinatedaliphatic and aromatic moieties are preferred.

Preferably, the optically transmissive relief forming polymer isselected to have as near as possible equal and opposite thermalexpansion and thermo-optic coefficients. The advantage of this is thatincreases in the optical path length (and hence phase change) due tothermal expansion of the material are compensated by decreases in itsrefractive index. This advantage requires that the optically activerelief is restrained from expanding laterally by the effect of thesubstrate material. This will be the case when the overlayer is small.“Temperature dependence of index of refraction of polymeric waveguides”,R Moshrefzadeh, M D Radcliffe, T C Lee and S K Mohapatra, J LightwaveTech, vol 10 (4), 420 (1992) describes a number of polymer materialshaving negative thermo-optic coefficients, positive thermal expansioncoefficients of the same magnitude. For example, PMMA has a thermo-opticcoefficient of −1.1×10⁻⁴ K⁻¹.

Preferably, the optically transmissive polymer has a refractive indexwhich is matched to the first refractive index, e.g. 1.51 at 633 nm whenthe first layer is Bk7 borosilicate glass or 1.46 at 633 nm when thefirst layer is quartz.

The refractive index of the optically transmissive relief formingpolymer may be modified by the inclusion of suitable additives into thepolymer. In particular the refractive index of the polymer may beadjusted by adding appropriate amounts of ethylene glycol dimethacrylatewhich can increase the refractive index (as measured at 1.32 or 1.55 μm)by an absolute value in excess of 0.02 when added at a level of 30% byweight.

Furthermore, an error in the depth of the optically active relieffeatures (compared to the designed depth) can be corrected by increasingor decreasing the refractive index of the optically transmissive reliefforming polymer by an equal fractional amount.

A further advantage of controlling the refractive index of the opticallytransmissive relief forming polymer is that the wavelength of operationof the MOE is shifted as a result. Hence a series of MOEs can beproduced from the same flexible dispensing layer so as to obtain an MOEwhich operates at high efficiency at the chosen wavelength. Changing therefractive index from 1.45 to 1.55 for an MOE designed to operate at 633nm for example would result in maximum efficiency operation at 677 nm.

The overlay of the relief forming polymer is reproducibly controlled toobtain a thickness appropriate to the function of the MRE and may, evenin those instances where a minimum overlay is desired, usefully serve toplanarise the receptive surface. In some instances, e.g. inmicro-mechanical devices, a relatively thick and uniform overlay may bedesirable for example to secure the relief forming polymer firmly to thefirst layer. In other instances, e.g. where the MRE is an MOE, it isdesirable to minimise the thickness of the overlay such that it does notinterfere significantly with the optical function of the MOE, i.e. theoverlay is optically insignificant. Preferably, the opticallyinsignificant overlay has a maximum thickness of less than 1.5 μm,preferably less than 1 μm, and particularly less than 0.5 μm over thesurface of the first substrate. The average thickness of the opticallyinsignificant overlay is preferably less than 1 μm and particularly lessthan 0.5 μm. The variation of the thickness of the overlay, whetheroptically insignificant or not, across the surface is preferably lessthan±0.75 μm, particularly less than±0.5 μm and especially lessthan±0.25 μm. This has the particular advantage of minimising wavefronterror.

The optical performance of the MOE depends on the phase differenceproduced between parts of the light beam which travel through differentareas of the surface relief pattern. The phase difference is defined bythe product of the depth of the features below the surface of the MOEand the refractive index of the material in which the MOE is produced.An advantage of having less than 1 micron of overlay between the firstlayer and the optically active relief is that this height is welldefined. Hence the MOE functions as designed. Also important is theflatness of the intervening surface between the optically active relieffeatures of the MOE. Improved performance results if the interveningsurface is flatter than the wavelength of the light being used. Withminimum overlay, the intervening surface is as flat as the first layeron which it is produced. Another advantage of minimum overlay is that itreduces optical loss of the part resulting from absorption of light bythe material by minimising the total thickness of material required todefine the surface relief pattern.

A very significant advantage of making polymer optically active relieffeatures on glass or another material with a low thermal expansioncoefficient is that the thermal stability of the MOE component isenhanced as a result of maintaining the pitch of such optically activerelief features and by minimising the volume of that material which hasa relatively high thermal expansion coefficient.

In order to facilitate the curing of the resin it is preferred to use aninitiator, for example a thermal and/or photoinitiator and particularlyan initiator which does not absorb light at the operating wave length ofthe MOE. Typically, when used, an initiator is present in the resin at aconcentration from 0.1 to 3.0% by weight, and preferably from 0.5 to2.0% by weight. Suitable photoinitiators include2-methyl-1-[4-(methylthio)phenyl)-2-morpholino propanone-1 (Irgacure907), 1-hydroxy-cyclohexyl-phenyl ketone (Irgacure184),isopropylthioxanthone (Quantacure ITX),Camphorquinone/dimethylaminoethylmethacrylate. Similarly a suitablethermal initiator is tert-butylperoxy-2-ethyl hexanoate (Interox TBPEH).

As the line of contact moves across the surface of the first layer theresin is effectively pushed across the surface and flows into the atleast one mould feature. The rate at which the line of contact advancesacross the surface will depend, amongst other things, on thecharacteristics of the resin. Typically, the resin has a viscosity from0.1 to 100 poise and more typically from 10 to 100 poise.

The resin may be fully retained within a mould feature as the line ofcontact moves from the mould feature, in which case the resin may becured at any convenient subsequent time. However, the resin may oftenshow some degree of resilience in the non-cured form in which case asthe line of contact moves from the mould feature the resin therein willtend to relax and exude from the mould feature. Where the relief featureis part of an MOE then this relaxation of the resin can reduce theeffectiveness of the MOE. To counter the relaxation of the resin it ispreferred that the resin is cured before the line of contact completelymoves from it.

Conveniently and preferably therefore, the resin contains aphotoinitiator which is activated by a particular wavelength of light,particularly UV light. A suitable source of light may then be used tocure the resin before the pressure applied along the line of contact isreleased and before the resin relaxes from the retaining feature. It isespecially preferred that the flexible dispensing layer is transparentto the light used and that the light is shone through the flexibledispensing layer towards the resin. In order to focus the lightsubstantially at the tip and thereby avoiding, for example, prematurecuring of the resin, the angle of incidence of the light onto the lineof contact may be required to be adjusted from polymer to polymer.Alternatively, for a given angle of incidence and where the first layeris at least partially transmissive to the light, the first layer may bechosen to have a thickness such that the internal refraction of theincident light acts to focus the light at the line of contact.Additionally, where the first layer is at least partially transmissiveto the light and is of a suitable thickness, a mirrored support may bepositioned under the first layer thereby causing the transmitted lightto be reflected back to the line of contact.

The pressure is applied along the line of contact by any suitable means.Suitably, the pressure is applied using an advancing bar or flexibleblade under a compressive load which may be drawn along the surface, orusing a roller under a compressive load which may thus on advancement orrotation retain the resin in the nip formed by the bar, blade or rollerbetween the flexible dispensing layer and the surface. It is thereforepreferred that the resin is cured at the nip as the line of contactprogresses across the surface.

The flexible dispensing layer is preferably a polymer film into whichthe mould features have been embossed. Such an embossed film ispreferably transparent to UV light, has high quality surface releaseproperties and is capable of remaining dimensionally sound during themoulding process. Conveniently, such an embossed film may be formed by(a) forming a master pattern having a contoured metallised surface whichconforms to the required relief structure, (b) electroforming a layer ofa first metal onto the metallised surface to form a metal master, (c)releasing the metal master from the master pattern, (d) repeating theelectroforming process to form a metal embossing master shim and (e)embossing the relief structure into a polymer film so as to form thedesired mould features.

Adventitiously, when transparent, the embossed film may be opticallyaligned so that the mould features may be precisely aligned on thereceptive surface of the first layer. Thus, the mould features may bemore easily oriented on the receptive surface, e.g. about a desired axisof or existing feature on the receptive surface. In particular, wherethe first layer is itself a lens then the optical axis of the lens maybe aligned with that of an optically active relief feature formed usingthe mould features such that the optical performance of the compositecomponent is optimised.

Additionally, the embossed film, if retained on the receptive layer, mayserve as a protective layer which can be removed at a later time.

A further advantage of making the MOE by the above method is that therefractive index of the relief forming polymer may be varied so as toimprove or modify the optical performance of the MOE. This is also abenefit because optical components with different operating wavelengthscan be made from the same master shim.

A further advantage of making the MOE by the above method is that themaster pattern can be made by a wide range of available techniques in awide range of materials and is not limited to being made in a materialwith good optical properties. For example the original master patterncan be made by direct electron beam patterning of photoresist,conventional photolithography, silicon micromachining (K E Peterson,Proc IEEE, Vol 70, 420 (1982)), laser beam writing (E C Harvey, P TRumsby, M C Gower, S Mihailov, D Thomas, Excimer lasers forMicromachining, Proc of IEE Colloquium on Microengineering and Optics,February 1994, digest No. 1994/043, paper 1; D W Thomas et al, Laserablation of electronic materials, European Mat Res Soc Monographs, Vol4, Ed. E Fogarassy and S Lazare, p221 (1992); H Schmidt, Micromachiningby lasers, Conf on Lasers and Electro-optics (CLEO EUROPE 94),Amsterdam, September 1994, Paper CMB1); plasma etching (D L Flamm inPlasma etching—an introduction ed by D M Manos and D L Flamm, AcademicPress Inc, London (1989), Chapter 2); and single point diamond turning.

A further advantage of making the MOE by the above method is that theflexible dispensing layer may be treated with any suitable material forany desired purpose, for example a masking or screening medium, apriming medium, or a medium conferring any desired optical, electrical,mechanical or fluid properties, such as ink, seed (catalyst) material, ametal precursor, an electrically conducting (precursor) medium, or abiological culture or the like which may be transferred by contactreproduction to the first layer or the overlayer as desired, for exampleto selected regions thereof on or about the relief features, using amodification of known techniques for example as described in Appl. Phys.Lett. 68(7), 1022-23

Moreover microlenses comprising relief features having a wide range ofaspect ratios, i.e. of height to width ratio, may be produced, forexample of aspect ratio up to 20, suitably up to 10 or up to 15depending on the relief forming polymer and the relief feature shape.

An advantage of fabricating an MOE in the form of a microlens array bythe above method is that the shape of the surface of each lens isdetermined by the mould and not by the fabrication process. This is incontrast with the conventional method of producing microlens arrayswhich relies on surface tension of a molten material to shape themicrolenses. The conventional method limits the maximum radius ofcurvature of each lens and hence the F-number of the lenses that can beproduced. The above method can be used to produce for example asphericlens shapes which give improved lens performance (less sphericalaberration).

A further advantage of fabricating a microlens array by the above methodis that a second optically functional surface or diffractive opticalelement, for example, can be formed on the surface of each of the lensesin the array at the same time as the lens itself is defined by use of amould having the appropriate surface profile or diffractive structure onits inner surface. Thus a profiled or combined refractive diffractivelens is produced. Such a combined lens performs a similar opticalfunction to an achromatic doublet lens (the combination of a lens ofnegative dispersion with one with positive dispersion).

A further advantage of the above method is that large areas of microrelief arrays can be produced at once, in particular microlens arrayswhich are often required for use as display screens. Micro relief arraysmay comprise repeating sections of identical or different relieffeatures.

Due to the sub-micron resolution of the above method, microlenses withsmall diameters and pitches may be produced.

A further advantage of the above method is that a set of substantiallyidentical structures may be produced. These may be used in associated orunassociated arrangement.

In optical systems which use microlens arrays there is sometimes arequirement for an optical element which consists of two identicalmicrolens arrays placed back to back, separated by a fixed distancerelated to the focal length of the microlens array and with the twoarrays aligned relative to one another. An advantage of the above methodis that because the same mould can be used to form each array, the twoarrays will be identical. Accurate separation of the two arrays can beachieved by controlling the thickness of the intervening first layer andthe focal lengths of each array can be adjusted by changing therefractive index of the second array until the distance which separatesthe arrays is substantially the sum of their focal lengths. Furthermore,because the method can use an optically transparent flexible dispensinglayer, the second microlens array can be accurately aligned on the backof the first layer by viewing through the flexible dispensing layer.

The concept and applications of fabricating arrays of light emittingdiodes with integrated diffractive microlenses fabricated by a differentmethod has recently been reported in “Arrays of light emitting diodeswith integrated diffractive microlenses for board-to-board opticalinterconnect applications: design, modelling and experimentalassessment”, B Dhoedt, P D Dobbelaere, J Blondelle, P V Daele, PDemeester, H Neefs, J V Campenhout, R Baets, Conference on Lasers andElectro-Optics (CLEO Europe 94), Amsterdam, 28 August to 2 September,paper CThI64 (1994). The above method may also be used with atransparent embossing film to form MOEs onto the surface of a substratewhich already has semiconductor devices which emit or detect light (e.g.laser diodes, light emitting diodes, photodiodes and vertical cavitylasers) such that the MOE features are accurately aligned with thesemiconductor devices.

The above method may also be used to produce MREs which are alignmentlayers for liquid crystal cells. Some types of liquid crystal material,in particular ferroelectric liquid crystals, require alignment layers inthe cell to orient the liquid crystal in a certain way. Conventionally,the alignment layer can be produced by physically patterning the glasssurface, for example by rubbing the surface in the required direction.Alternatively, a thin layer of a material such as MgF₂ is evaporatedonto the surface. The purpose of this alignment layer is to align theliquid crystal material with a small tilt relative to the normal to thesurface. By varying the angle of evaporation, the angle of the tilt canbe varied. The current drawback of this method is that the surface areais limited by the size of the evaporator's chamber. An advantage of theabove process is that a larger surface area may be structured using anembossed film prepared from several master shims. Alternatively,alignment structures for liquid crystals may be made for example in theform of a plurality of high aspect ratio MRE's resembling relief “hairs”of the order of 200 nm high and 20 nm wide. Adventitiously, the abilityto minimise the overlay is that there is less material covering theelectrode which is used to apply an electric field to the liquid crystalcell thereby potentially resulting in lower switching powers.

The present invention is illustrated in non-limiting manner by referenceto the following figures.

FIG. 1 shows a section of the image produced by a 16×16 MOE beam arraygenerator.

FIG. 2 shows the variation in intensity with temperature for a 4×4 beamarray generator.

FIG. 3a shows a part of a nickel shim for preparing mould features in aflexible dispensing layer to be used to produce an MOE.

FIG. 3b shows a part of the MOE produced from the flexible dispensinglayer prepared using the nickel shim shown in FIG. 3a.

FIGS. 4a and 4 b are SEMs showing a variety of surface reliefs.

FIG. 5 is an SEM of a relief feature in the form of a microlens array.

FIG. 6 is Tencor Alpha-step surface profiling machine trace showingoverlayer thickness of an MOE.

FIG. 1 was produced from an MOE described in Example A.

In FIG. 2, line (1) represents the variation in temperature that the 4×4beam array generator underwent as described in Example A. Line (2)represents the optical response of the equipment without any samplebeing present. Line (3) represents the optical response of the MOEfabricated on glass. Line (4) represents the optical response of the MOEfabricated on film. Line (5) represents the optical response when anarea of PET film with no MOE on it was illuminated.

FIG. 3a shows part of a nickel shim as used in Example B.

FIG. 3b shows part of microlens array produced according to Example Bfrom a flexible dispensing layer in which the mould features had beenformed using the nickel shim shown in FIG. 3a.

FIGS. 4a and 4 b show the various MREs produced in Example D.

FIG. 5 shows a hexagonal microlens array of 125 micron pitch and 204micron focal length in air as produced in Example E.

FIG. 6 was produced from an MOE described in Example F. In region (1)the polymer film was removed from the glass to provide a referencelevel.

The present invention is further illustrated in non-limiting manner byreference to the following examples.

Preparation of a Flexible Dispensing Layer in the Form of an EmbossedFilm EXAMPLE 1.1

The following example describes the preparation of an embossed polymerfilm having a release treated surface.

A wet coating of neat fluorinated dimethacrylate resin thickness 20 μmwas applied to a 100 μm thick polyester substrate (Melinex grade 506).The coating was partially cured by exposing it to UV irradiation for 2 s(whilst in air) from a Fisons F300 ultra violet lamp system delivering300W/inch.

The coated polyester was then fed into a nip between a 400 mm diametersteel roller which carried a nickel embossing shim containing surfacerelief microstructures (e.g. 125 μm pitch microlens arrays) and a 150 mmdiameter roller faced with silicone rubber of hardness 70 shore. Thecoated polyester entered the nip such that the coated side was loadedagainst the shim. The nip load was controlled to 159 kg (350 lb) over aface width of 400 mm. The speed of the 400 mm diameter drum was set to3.3 cm.s⁻¹.

On exiting the nip, the coated polyester and nickel shim passed througha UV source as described above which fully cured the coating whilst incontact with the shim to form the embossed polymer film. The embossedfilm was then stripped away from the nickel shim and was then baked at80° C. for 16 hours in an oven.

A release layer of release material, Freekote FRP (Dexter Corporation),was applied to the embossed film by washing with a solution of releasematerial and then drying with compressed air. This process was repeatedfour times.

EXAMPLE 1.2

The following example describes the preparation of an embossed filmwhich contains an internal release material.

A wet coat of 20 μm was applied to 100 μm polyester substrate (Melinexgrade 506) from the following formulation:

97.5 parts Ebercryl 150 (epoxy acrylate ex UCB Ltd.)

2.5 parts Ebercryl 350 (silicone acrylate ex UCB Ltd.)

20 parts LG156 (PMMA)

2 parts Irgacure 651

mixed in solution 20% w/w in MEK. This resulted in a dry thickness of 20μm.

This coated substrate was processed in the same way as described in theExample 1.1, apart from the baking and the subsequent application of arelease material.

EXAMPLE 2.1

The following example describes the preparation of MREs on a rigidsubstrate using the previously prepared embossed film according toExample 1.1.

A rigid glass substrate was prepared by washing thoroughly in a 30%Dekon 90 solution in water, a hot water rinse, an acetone wash andfinally a wash with isopropanol. The substrate was then dried in an ovenat 150° C. for 15 minutes.

The substrate was then positioned on a flat assembly bed and secured byvacuum.

The assembly bed was provided with the means to traverse a 75 mmdiameter rubber covered nip roller along the length of the assembly bed,which forms an advancing nip region into which a UV source was focused.

An embossed film as described in Example 1.1 was placed face down on topof the glass substrate and anchored at one end with a single sidedadhesive tape.

A quantity of resin (LUXTRAK 0208), sufficient to fill the mouldfeatures in the embossed polymer was placed between the glass substrateand the embossed polymer as a bead across the direction of travel of theassembly bed and at the anchored end of the embossed film. Thetraversing roller was then positioned 3 mm before the bead of resin andhad a downward load of 40 kg across a face width of 80 mm applied.

The UV source was powered up and the nip roller was advanced at the rateof 1 cm.s⁻¹ along the assembly bed across the embossed film/glasssubstrate. The resin was squeezed into the mould features and was curedby the UV source. After curing the embossed polymer was peeled awayleaving the cured resin affixed to the glass substrate. 100% transferwas achieved although some witness marks of residual release materialwere apparent.

EXAMPLE 2.2

The Example of 2.1 was repeated using the embossed film as prepared inExample 1.2, the LUXTRAK resin as described in Example 2.1 and afluorinated dimethacrylate resin of formulation:

Fluorodimethacrylate: 97 wt %

Photoinitiator (Irgacure 651): 2 wt %

Thermal initiator (Interox TBPEH): 1 wt %

100% transfer was achieved for the LUXTRAK resin and about 80-90% forthe fluoropolymer resin. There were no witness marks apparent.

EXAMPLE A

Using the method as described in Example 2.2, a number of syntheticMOEs, traditionally known as computer generated holograms, werefabricated to a depth 0.6 micron and a smallest lateral dimension of 1.5micron in LUXTRAK LCR 0208 on a glass substrate.

The chosen MOE was designed to produce an array of spots of nearly equaloptical power in the far-field behind the element when it wasilluminated with a laser beam of wavelength 670 nm. The laser beam wasderived from a diode laser but could have been produced by another typeof laser source.

The fabrication of the master pattern was as described in “Syntheticdiffractive elements for optical interconnects”, M R Taghizadeh and JTurunen, Optical Computing and Processing, vol 2 (4), p221-242, 1992. Itconsisted of a binary (2 level) surface relief structure produced in aquartz wafer. The diameter of the wafer was large enough to allow 12MOEs, each of size 15 mm by 15 mm to be defined onto the one wafersurface. The surface of the quartz master was rendered conducting byevaporating on a 10 nm thick layer of chromium followed by a 60 nm thicklayer of silver. A nickel shim was then grown from the quartz master byan electroforming process.

The functions of each of the MOEs produced were 2×2, 4×2, 4×4, 8×8,8×16, 16×16 and 16×32 beam array generators. FIG. 1 shows the patternproduced when the 16×16 MOE was illuminated by the beam from a diodelaser at a wavelength of 676 nm. This image was captured using aElectrophysics Micronviewer vidicon camera connected to an image capturesystem.

The intensity of one of the beams in the first order diffraction patternof a 4×4 beam array generator MOE fabricated in LUXTRAK LCR 0208 resinon a glass substrate as a function of temperature was compared to thatof the same MOE fabricated in urethane acrylate (Harcoss resin 6217) ona “Melinex” film substrate. FIG. 2 shows the results of this experiment.The diffracted beam from the MOE fabricated on the film varied by up to10% over the temperature range 25° C. to 85° C. In comparison, thediffracted beam from the MOE fabricated on the glass varied by only afew percent. A large variation was also observed when the beam passedthrough the film but outside of the patterned area. This indicates thatthe thermal mechanical behaviour of the substrate has a strong effect onthe performance of the MOE.

EXAMPLE B

Example A was repeated except that the MOE fabricated acted as amicrolens array. In this example, the original master was produced bydirect electron beam writing of photoresist followed by dry etching ofthe pattern into quartz. The MOE contained 16 levels of surface relief(16 phase levels) so as to approximate more exactly a continuous surfaceprofile. The advantage of this is that the optical efficiency of the MOEis higher than the equivalent binary phase MOE. As a result of the extraphase levels the smallest lateral feature size in the surface relief wasabout 200 nm. This is significantly less than the smallest lateralfeature size on the binary surface relief. The UV embossing process usedto manufacture the MOEs has the capability to reproduce accurately thevery small features required. This is a significant advantage over othertypes of embossing methods (e.g. hot roll embossing or injectionmoulding). FIG. 3 shows for comparison an 800 micron aperture microlenson the nickel shim and the same lens formed in 2 micron thick urethaneacrylate resin (Harcross 6217) on 100 micron thick ICI Melinex film.

EXAMPLE C

Example A was repeated except that the Micro-Optical Element (MOE)fabricated was a surface relief diffraction grating of period 1.1 μm(smallest feature size 0.55 μm) and depth 130 nm. The grating patternconsisted of an annulus of about 30 mm diameter and about 2 mm width.The surface of the MOE was coated with 70 nm of Aluminium by evaporationso as to render it highly reflective. The surface of the grating wasilluminated through the glass substrate using light from a He—Ne laserat 633 nm. The irradiance of light reflected from the grating into oneof the first diffraction orders was measured and compared to the amountof light reflected from an adjacent metallised area on the MOE where nograting was present. This ratio, also known as the efficiency, was foundto be 39±0.5%. The experiment was repeated using two other samplesmanufactured in the same way. Their diffraction efficiencies weremeasured to be 39±0.5% and 37±0.5% respectively. The efficiency isdirectly related to the accuracy with which the replication processreproduces the period and depth of the grating structure. Poorreplication of the surface relief results in efficiencies of less than10%.

The thickness of the overlayer was measured using a Tencor Alpha-stepsurface profiling machine. The thickness was found to be 0.5 μm.

The experiment was repeated with the same grating pattern but using apiece of the internal release coated “polymer shim” (described inexample 1.2) used in the manufacture of the replica on glass (i.e. theprevious sample). Once again the sample was arranged so as to read outthrough the substrate. The diffraction efficiency was measured to be37%.

This experiment shows that there is no measurable reduction inefficiency due to the use of the polymer shim intermediate.

The experiment was repeated with the same grating pattern but using asample manufactured by coating a 2 μm thick coating of urethane acrylate(Harcross 6217) onto 175 μm thick PET film (ICI MELINEX) and UVembossing. The diffraction efficiency was measured to be 36±0.5%. Thisexperiment shows that there is no reduction in efficiency by formulatingthe polymer shim material so as to contain an internal release agent.

The experiment was repeated with the same grating pattern but using 0.5mm thick polycarbonate sheet (LEXAN) as the substrate. The diffractionefficiency was measured to be 36±0.5%. This experiment shows thatalternative rigid substrate materials can be used and that there is nosignificant reduction in efficiency from the resultant parts.

In all of the experiments above, the efficiency of the MOE produced inthis example is significantly larger than that measured from the samesurface relief grating structure manufactured from the same masternickel shim by comparative techniques of hot embossing (efficiency 11%)and injection moulding (efficiency 4%).

EXAMPLE D

Using the method previously described in Example 1.1 an embossed filmwith a number of continuous surface relief microstructures wasfabricated. The structures included 12 micron high staircases, pyramidsof varying size, grooves, tracks, slopes, hemispherical structures andwells. FIG. 4 shows an SEM photograph of some of the structures formedaccording to Example 2.2 as LUXTRAK LCR 0208 on glass. Being able toprepare such deep relief features is an advantage because more phaseinformation can be impressed onto light which diffracts from it andhence the optical function of the relief features are enhanced. As shownin FIGS. 4A and 4B, the relief features 4 protrude from the overlay 3.

EXAMPLE E

A nickel embossing shim was made by the following method:

A 100 mm square piece of glass was cleaned and dried. The glasssubstrate was placed in a vapour bath of Shipley Microposit primersolution for 2 minutes to improve the adhesion of the subsequentphotoresist layer. AZ4562 photoresist was spin coated onto the glasssubstrate at a speed of 2000 rpm for 20 s and the sample softbaked for10 minutes at 90° C. on a hotplate. The thickness of the photoresistlayer was measured to be 9.9 micron using a Tencor alpha-step machine.The sample was exposed for 35 s by contact through a photomaskcontaining a pattern of 125 micron pitch microlenses of diameter 120microns. The resist image was developed for 7.5 minutes in a 1:4 mixtureof AZ developer solution and water. The exposure and developmentconditions were chosen to ensure that all the photoresist had beenremoved between each microlens island. Finally, the microlenses wereformed by placing the sample onto a hotplate at 15° C. for 45 s. By sodoing, the resist material was caused to melt and surface tension drewthe resist islands into hemispherical microlenses.

The surface of the microlens sample was rendered conducting byevaporating thin chromium and silver layers onto it. A nickel master wasthen electroformed from the sample. The nickel master was used to growan embossing shim which was used to produce an embossed film asdescribed previously.

Using the laminating method previously described, a micro-optical lensarray as shown in FIG. 5 was fabricated on a 2 mm thick glass substrateusing fluorinated dimethacrylate resin. As shown in FIG. 5, the lensesare relief features 4 which protrude from the overlay 3.

EXAMPLE F

A microlens array was fabricated on a 1.1 mm thick borosilicate glasssubstrate (B270 glass) using the embossing shim whose preparation wasdescribed in example E. The material used was Luxtrax LCR 0208 UV cureacrylate resin. The optical properties of the resin on glass replicawere measured so as to compare its optical performance to that of theoriginal melted photoresist lenses. The focal length over the 70 mm by70 mm area of the microlenses was found to be 204.4 μm with a standarddeviation of 1.5 μm. The Strehl ratio was measured to be 0.82 (a Strehlratio of 1 indicates diffraction limited performance). The lens shapewas found to show only 0.55 of a wavelength deviation from sphericalwhen illuminated with light at 633 nm. These parameters are comparableto those measured for similar melted photoresist microlenses, showingthat the aberrations are introduced not during the replication process,but are faithfully reproduced from aberrations present in the meltedphotoresist microlenses.

The thickness of the overlayer on this sample was measured using aTencor Alpha-step surface profiling machine. The trace obtained is shownin FIG. 6. The thickness was found to be less than 0.4 μm. (Note therelief structure height returns to the level which is bare glass(polymer has been removed to glass for reference purpose adjacent theboundary relief structure)). As shown in FIG. 6, the bare glass is thesubstrate 1, the overlay 3 is a thin layer of polymer on the substrate1, and the relief features 4 are the lenses.

EXAMPLE G

The Nickel embossing shim and method described in example E was used tofabricate microlenses on the planar side of 25 mm diameter plano-convexglass lenses. In order to locate the lenses stably during theembossing/laminating process, they were mounted in an array in apolypropylene mounting plate with recesses machined into it using a toolof the same radius of curvature as the lenses. The use of thetransparent polymer shim enabled the embossed pattern to be accuratelycentred on the individual lenses. A further advantage of this method isthat the resultant part does not require any further cutting.

EXAMPLE H

The method described in Example E was used to fabricate a microlensarray on a 300 μm thick glass substrate. A substrate of this thicknesswas chosen so that the focal plane of the microlens array would coincidewith the back surface of the glass substrate. The focal length of thelens array in glass is equal to the focal length in air (204 μm)multiplied by the refractive index of the substrate (approx 1.5 in thiscase). Small changes in the focal length could have been made bychanging the refractive index of the polymer resin by adding an indexmodifier to the formulation. However, this was not required in thisexample as the focal length in glass was approx 300 μm.

Two samples were made and the lens arrays placed so that their uncoatedsides were in contact. Upon aligning the arrays so that the microlenseswere overlayed on top of each other, the combined lens arrays acted as a1:1 relay lens and were able to image objects placed beneath them.

Thin glass is difficult to handle and breaks easily therefore it wouldbe difficult to have achieved this example by using a process whichrequires a high load.

EXAMPLE I

A microlens array was prepared in the same way as detailed in Example E.The microlens array was then positioned on a flat assembly bed andsecured by vacuum such that the microlenses were on the top surface. Theassembly bed was provided with the means to traverse a 75 mm diameterrubber covered nip roller along the length of the assembly bed, formingan advancing nip region into which a UV source could be focused.

A polyester laminating substrate, “Melinex” grade 400, was placed on topof the microlenses and anchored at one end with a single sided adhesivetape. A quantity of resin with a different refractive index, in thiscase a fluorodimethacrylate with 25 wt % added ethylene glycoldi-methacrylate, sufficient to encapsulate the microlenses, was placedbetween the microlenses and the polyester laminating substrate in a beadacross the direction of travel of the assembly bed and at the anchoredend of the laminate.

The traversing roller was then positioned 3 mm in front of the bead ofresin and a downward load of 40 kg applied across a face width of 80 mm.The UV source was switched on and the nip roller advanced at the rate of0.6 m.minute⁻¹ along the assembly bed across the laminate. The resinfilled the cavities formed between the microlenses and laminatingsubstrate and was cured by the UV source. After curing the laminatingsubstrate was peeled away.

The purpose of this operation was to immerse the microlenses in thehigher index material so as to increase the focal length of themicrolenses compared to their focal length in air.

EXAMPLE J

An embossed film carrying a 500 micron pitch microlens array wasprepared using the method described in Example 1.1. The nickel embossingshim was selected to be a male so that the embossed film was female. Theembossed film was coated with ink so that ink was transferred to theintervening, areas extending between the mould features. The embossedfilm was then used to prepare the microlenses as before. At the sametime as forming the microlenses on the glass substrate, ink wastransferred to the unoccupied glass surface between the microlenses.

The advantage of this process is that cross-talk between the microlensesis reduced when the lenses are used in an optical system.

What is claimed is:
 1. A composite micro relief element for use inmicro-optical, micro-fluidic, micro-electrical, micro mechanical ormicro-chemical applications which comprises (a) a first layer of a firstsubstrate, the first layer having a receptive surface; and (b) a layerof a relief forming polymer which is retained on the receptive surfaceof the first layer, wherein the layer of relief forming polymercomprises overlay and at least one relief feature portions; wherein theoverlay is affixed to the receptive surface, serves to planarise thereceptive surface, and has a thickness variation less than +/−0.75 μm;and wherein the least one relief feature protrudes above the overlay. 2.A micro-relief element according to claim 1 which is a micro-opticalelement, wherein (a) the first substrate is optically transmissive,having a first refractive index; (b) the relief forming polymer isoptically transmissive and has a second refractive index which is thesame as or different from the first refractive index, and the overlayhas an optically insignificant effect having a maximum thickness of lessthan 1.5 μm; and (c) the at least one relief feature is opticallyactive.
 3. A composite micro relief element according to claim 1 whichcomprises additionally (d) a second layer of an optically transmissivesecond substrate having a third refractive index which is superimposedupon the at least one optically active relief feature and wherein notall of the first, second and third refractive indices are the same.
 4. Acomposite micro relief element according to claim 3, wherein the secondlayer is provided with at least one mold feature.
 5. A micro reliefelement according to claim 1 which comprises additionally a supportsubstrate which is associated with and supports the first layer.
 6. Acomposite micro relief element according to claim 1 wherein thereceptive surface of the first layer and/or the overlay comprises acoating of an agent or material selected or adapted to confer bonding,internal release, anti-reflection, heat dissipation, thermal expansion,thermo-optic, electrically conducting, optically modifying, and/orreflection properties.
 7. A composite micro relief element according toclaim 1 which is a large area micro relief array, wherein said at leastone relief feature which protrudes above the overlay comprises aplurality of relief features.
 8. A micro relief element according toclaim 7 comprising repeating sections of identical or different relieffeatures.
 9. A composite micro relief element according to claim 1wherein said at least one relief feature which protrudes above theoverlay comprises one or more continuous, stepped or other wise optical,fluidic, electrical or mechanical structures and wherein said at leastone relief feature has an aspect ratio of up to
 20. 10. A set of microrelief elements comprising a plurality of elements as defined in claim 1which are substantially identical, and are in associated or unassociatedarrangement.